Yes it is legal. You have bought the yeast (like buying a dog) and breeding the yeast is your prerogative (like breeding a dog). I have not heard of any patented yeast strains, although someone could patent their yeast strain if it was genetically modified (although even this is being challenged in court right now). So you could even sell the yeast if you so chose, but you may not use any trademarked or copyrighted terms used by the brewery who made the beer in marketing your yeast.

Second, as mentioned before, not all beers have live yeast in them from which you might get a culture.

Third, some breweries bottle with a yeast different than they ferment with, so it is possible that you'd end up culturing a stain that does not have the profile you are looking for.

All that said, if you have a bottle conditioned beer that you'd like to culture yeast from, you'll want to get a petri dish with agar, an inoculating loop and an alcohol lamp. Sterilize the loop in the alcohol flame, dip it into a fresh bottle of beer (the yeast will be in better condition) and run it lightly through the agar in a zig zag pattern.

Let this sit for a while. You should see some pure white colonies form. The should be growth from a single or few cells of the yeast. You could then use to build up a starter in several stages (you'll need to start with just a few MLs of unfermented wort to build up the cell count) to get large enough to pitch in a beer.

I think you guys are making it way more difficult than it need be. I made a Sierra Nevada Pale clone. I took one bottle of SNPA, let it sit in the fridge for a few days, then carefully decanted all but 1 inch of beer. I then boiled two tablespoons of DME in 6 oz of water, cooled it and added to the bottle. Added an airlock and within a few days had an active culture. I ramped it up in a growler a few days before brew day. Worked great.

I too use the bottle in which the beer came as my starter bottle... just add some wort. It's as close to sterile as your starter flask is going to be, right?

I successfully revived the yeast from an Orval bottle this way. Out of the 12 bottles of Orval, 6 didn't start, a few smelled bad, a couple smelled like regular yeast, and one had that magical Orval aroma!